Improved Performances of Micro-electromagnetic Energy Harvesting Devices by Minimizing the Demagnetization Field

Abstract

The potential of wirelessly connected smart world of ‘Internet of Things’ technological platform is restricted by the lack of ambient energy sources capable of powering the sensors perpetually[1],[2]. This issue has surged the research to investigate the prospect of harvesting the energy out of ambient mechanical vibrations[3–5]. Among the different vibrational energy harvesting (VEH) transduction mechanisms, electromagnetic (EM) transducers are the most promising one. However, the difficulty in miniaturisation and integration of the high performance permanent magnets in MEMS scale devices hinders the continuous demand of increasing power density (=power/device volume). In general, CMOS compatible development of high energy product (BH max permanent magnets with thickness of the order of microns to hundreds of microns is a key challenge for a number of magnetic MEMS applications including VEH[6–7]. But the problem extends beyond just high energy product magnetic material deposition and relates to the lack of intelligent design strategies. When a relatively thin film/block of permanent magnet is used in a MEMS device as source of magnetic field, the stray magnetic field appears only from the edge of the magnet and a large part of the material is wasted. This is due to the presence of the demagnetization field which acts to demagnetize the magnet in a direction which is opposite to the direction of the magnetization[8]. Hence, the magnetic flux intensity is greatly reduced which affect the performance of integrated magnetic transducers. Here, we propose to replace a block of permanent magnet by micro-patterned array of magnets, diminishing the demagnetization effect and enhancing the magnetic stray field. In that case, the magnetic flux density can be intensified over a small space due to increase of the edges of magnetic elements, which is shown quantitatively in Fig. 1(a) using FEM simulation in COMSOL. Thus electromagnetic coupling co-efficient can be substantially improved resulting in higher output power. Different patterned structures are simulated to derive the optimized configuration to be used in MEMS based EM VEH devices. To demonstrate the potential of the proposed approach, micro-patterns of the Co-rich CoPtP permanent magnets are developed at room temperature using a combination of standard lithography and an optimized pulse reverse electrodeposition techniques. Among different deposition methods[9],[10], electrochemical route is an attractive choice due to its low cost and relatively high deposition rate at CMOS compatible temperature. Compared to the conventional DC plating, significant improvements in the microstructure of the developed thick CoPtP micro-magnets are obtained using the pulse reverse electroplating technique which improves the hard magnetic properties as well. Up to $\sim 30 \mu \mathrm{m}$ thick patterned structures are developed with intrinsic coercivity >3 kOe and maximum energy product of 45.9 kJ/m3. In order to demonstrate the substantial advantage of optimized, micro-patterned magnetic structures compared to a block of integrated magnet, a novel device topology of micro EM VEH is adopted as shown in Fig. 2(a). The device consists of assembled components such as micro-fabricated silicon spring structure (natural frequency = 500 Hz), double layer electroplated copper coil with 144 turns and 190 ohm internal resistance and different micro-patterns of CoPtP magnets. The magnetic arrays are placed only above one side of the coil so that the later can move from a region of high magnetic flux density to zero flux density, generating large flux gradient vis-a-vis induced voltage. By changing the magnetic structure from block to square patterned magnet the electromagnetic coupling co-efficient increases from 0.2 mWb/m to 1.6 mWb/m, as obtained from FEM simulation. Experimental results show (Fig. 2(b)) that under optimized load condition, the device with square micro-patterned magnets produce 4 times higher output power compared to the same produced by a device with an entire block of magnet of similar footprint. In conclusion, this work reports a novel integration strategy to incorporate rare-earth free, CMOS compatible micro-magnets in fully integrated MEMS based EM VEH device in order to improve the device performance significantly, thereby, making it a suitable candidate for powering the sensors within Internet of Things technology. Further improvements can be obtained by developing higher aspect ratio, highly anisotropic magnetic structures which can be attributed to future works.